Can I also suggest having a think about what he sees as normal at home. I'm guessing that, like in my family, you don't actually ask permission before you hug/kiss a family member? Maybe it'd help if you modelled the behaviour for him until her learns. So he sees mum and dad:

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ask for a hug

wait for consent

then give the hug

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and also

ask for a hug

get told no

don't then follow though with a hug - maybe come up with something he can offer a person instead, so if someone turns down a hug or kiss he can offer to play or do something else instead so its not just a dead end when he hears no from someone when he offers affection?

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You can see if will practice with you, you ask him before you hug, he has to ask you before you hug.

Not sure if this would be overkill though so I'd be interested to hear if others think this would help?